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The First Report on Chemoselective Biguanide-Catalyzed Henry Reaction Under Neat Conditions

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The First Report on Chemoselective Biguanide-Catalyzed Henry Reaction Under Neat Conditions 3640 Bull. Korean Chem. Soc. 2012, Vol. 33, No. 11 Abdolhamid Alizadeh et al. http://dx.doi.org/10.5012/bkcs.2012.33.11.3640 The First Report on Chemoselective Biguanide-Catalyzed Henry Reaction under Neat Conditions Abdolhamid Alizadeh,†,‡,* Mohammad M. Khodaei,†,‡,* Gisya Abdi,† and Davood Kordestani† †Department of Organic Chemistry, Faculty of Chemistry, Razi University, Kermanshah 67149, Iran ‡Nanoscience & Nanotechnology Research Center (NNRC), Razi University, Kermanshah 67149, Iran *E-mail: [email protected] (A. Alizadeh); [email protected] (M.M. Khodaei) Received May 23, 2012, Accepted August 11, 2012 An efficient synthetic method for direct Henry reaction catalyzed by a biguanide; namely metformin, as an organosuper-base, between a variety of aromatic and aliphatic aldehydes and nitromethane under neat conditions has been developed. Convenient procedure for removal of the catalyst, chemoselective acquiring of β-nitroalcohols as predominant products, as far as possible short reaction time with excellent conversions are advantages of the developed protocol. Key Words : Organosuper-base catalysts, Henry reaction, Nitroaldol reaction, Biguanides, Metformin Introduction dines and guanidines, have been attracting much attention in organic synthesis due to their potential functionality.10a-d One One of the main purposes in organic chemistry is carbon- of the important and beneficial characteristics of an organic carbon bond formation and in this regard, the Henry base, especially from the view point of environmental as- reaction1 (an aldol-type C-C bond formation; called nitro- pects, is the ability of recycling use in repeated reactions, in aldol reaction) has been used extensively in many important which reversible proton transfer occurs between the base synthetic strategies.2 This reaction simply takes places with and a substrate as an acidic counterpart. Thus, powerful the help of basic organic,3-5 inorganic catalysts6 or quaternary organic bases that may be applicable in various organic ammonium salts.7 These catalysts often result in side reac- syntheses as basic catalysts have attracted much attention. tions such as aldol-condensation, Cannizzaro reaction,8 Biguanides with a dual guanidine-like moiety are interest- nitroalkene formation, Michael reactions, retro-aldol reac- ing class of organosuper-bases in which their basicity is due tion1 and Nef reaction.9 Therefore the development of new to the construction of a highly effective conjugation system catalysts for the Henry reaction to avoid these side reactions after protonation under reversible conditions; primitively, it and likewise produce nitroalchohols chemoselectively in is a reflection of the number of canonical forms, especially short time, under mild conditions and using inexpensive and isoelectronic forms, in the resonance system (Scheme 1). less toxic solvent is highly desirable. Generally, biguanides that contain three amino groups are Recently, nitrogen-containing organobases, such as ami- stronger bases than guanidines and amidines that have two Scheme 1. Structures of amidines, guanidines and biguanides. The number of canonical forms, in the resonance system of protonated metformin constructs a highly effective conjugation system. The First Report on Chemoselective Biguanide-Catalyzed Henry Reaction Bull. Korean Chem. Soc. 2012, Vol. 33, No. 11 3641 and one amino groups, respectively.11 To the best of our Table 1. Optimization of the reaction condition knowledge, whereas nitro-aldol reaction has been well studied with amines,3 amidines4 and guanidines,5 there is no report on employing the biguanides in nitro-aldol reaction. In this study, for the first time, we have utilized metformin, Cat. CH3NO2 Time Conversion/NMR an easily available biguanide and very strong organosuper- Entry Solvent base, to catalyze the Henry reaction of a variety of aliphatic, (mmol %) (equiv) (h) yield (%) aromatic and heteroaromatic aldehydes with nitromethane at 1- 2 -240 room temperature without addition of stoichiometric amount 2102 - 2 85 of the base. 3 5 2 - 1 100/100 42 2 -2 45 Results and Discussion 55 1 -2 50 6104 - 2 90 Our investigations on the chemoselective synthesis of 75 2H2O2450 nitroalcohols from various aldehydes and nitromethane as a 85 2EtOH2450 pro-nucleophile in the presence of catalytic amount of met- 95 2CH3CN 24 60 formin, began with the optimization of the reaction condi- 10 5 4 H2O2460 tions. The synthetic pathway is shown in Scheme 2. Our initial examination was involved to investigate the reaction of 2,4-dichlorobenzaldehyde with nitromethane in nitroalkene, aldol-condensation, Cannizzaro and Nef reac- various conditions. Table 1 lists the representative data tions. obtained for the synthesis of 2-nitro-1-(2,4-dichlorophenyl)- With a reliable set of conditions in hand, and in order to ethanol under various experimental conditions. The reaction investigate the utility of metformin as an appropriate basic mixture was stirred at room temperature and the progress catalyst in Henry reaction, we also probed the scope and was monitored by TLC. A control experiment in the absence generality of the reaction of several aryl and alkyl aldehydes of metformin (Table 1, entry 1) showed no evidence of with a variety of functionalities (Table 2). As shown in Table nitroalcohol formation confirming the basic role of met- 2, β-nitroalcohols were obtained in good to excellent con- formin as the organosuper-base catalyst in the described versions and the yields of the desired Henry products were transformation. estimated using the crude 1H NMR spectra. In addition, various amounts of metformin were added It was found that aromatic aldehydes with electron-with- under the mentioned conditions. Among different amounts drawing groups in reaction with nitromethane give the of the catalyst and nitromethane (Table 1, entries 2-6), the desired products with excellent conversions (Table 2, entries best results were achieved when the reaction was performed 2-5). When the reaction was performed with halo-benz- with 5 mol % of catalyst and 2 equiv. of nitromethane to aldehydes (Table 2, entries 6-10), the corresponding 2-nitro- give 100% conversion of the starting materials to the related alcohols were obtained in good to excellent conversions β-nitroalcohol product (Table 1, entry 3) and the crude 1H ranging from 82% to 100%. Although aldehydes with elec- NMR spectrum was employed to estimate yield of the desir- tron-donating groups are more unfavorable to react rather ed Henry product which was ~100%. In addition, various than those with electron-withdrawing groups however they solvents were employed for the aforementioned reaction gave the desired products in high to excellent conversions (Table 1, entries 7-10) and high conversion was observed (Table 2, entries 11-13) which showed that the nature of the using nitromethane as neat conditions without the presence substitutions on the ring did not significantly hamper the of solvent. Hence, the optimized conditions found for the reaction. synthesis of 2-nitroalcohols starting from aldehyde and In addition, reaction of di-aldehydes such as iso- and nitromethane are the use of 1 equiv. of aldehyde, 2 equiv of terephthalaldehyde with nitromethane under the developed nitromethane in the presence of 5 mol % of metformin at conditions led to the related di-β-nitroalcohols (Table 2, room temperature stirring for 1 h under aerobic conditions. It entries 14, 15) through double cascade Henry reactions. is worthy to mention that 1H NMR studies showed no Interestingly, heteroaromatic aldehydes reacted with the evidence of by-product formation resulting from typical side nitromethane and gave their related β-nitroalcohols with reactions such as dehydration of the 2-nitroalcohol into 100% conversions (Table 2, entries 16, 17). Due to the possible self-condensation reaction of aliphatic aldehydes, they seem to be less reactive in Henry reaction. Notably, under the developed protocol the biguanide-type catalyst was capable of catalyzing the Henry reaction of an aliphatic aldehyde with nitromethane and chemoselective acquiring of β-nitroalcohols in high to excellent conversions Scheme 2. Henry reaction of nitromethane with carbonyl compounds (Table 2, entry 18). catalyzed with metformin under neat condition. The scope of the developed procedure was more expand- 3642 Bull. Korean Chem. Soc. 2012, Vol. 33, No. 11 Abdolhamid Alizadeh et al. Table 2. Henry reaction between various aldehydes (1 mmol) and nitromethane (2 mmol) using 5 mol % of catalyst at room temper- ature for 1 h Conversion NMR Yield Entry Aldehyde (%) (%) 1 100 ~ 100 b Scheme 3 2100− ed, especially on ketone substrates. In order to evaluate the 3100b − possible catalytic activity of biguanide in the Henry reaction of ketones, we probed the reaction of various aliphatic, aromatic, and aryl-alkyl ketones with nitromethane. Among 4100b − the studied ketones, only cyclohexanone showed the excellent reactivity in short time (Table 2, entry 19) while 595− acetone, acetophenone and benzophenone failed to give the desired products under the developed condition (Table 2, 69590entries 20-22) possibly due to the preferential self-condensa- tion reaction or retro-Henry reaction.12 The Henry reaction with ketones is sensitive to steric factors and generally gives 7 100 100 a complex mixture of products depending on the ratio of reactants, base, temperature and time. 88888To provide more experimental supports for the above- mentioned chemoselectivity, we mixed 1 mmol of acetone,
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